Dark comet
Dark comets are a class of asteroids that exhibit some behavior consistent with comets, but do not grow a comet's tail or produce a coma, making them visually appear as asteroids. They are distinguished from other asteroids by their non-gravitational acceleration, which causes them to deviate from their expected orbital path. Dark comets were first identified as a distinct class of objects in 2023.[1] They mostly fall into two families, the outer dark comets, and the inner dark comets, which are distinguished by their size and orbital characteristics.[2]
Discovery
[edit ]The first object that exhibited "dark comet" behavior was ‘Oumuamua, an interstellar object discovered in 2017, which was found to exhibit "non-gravitational acceleration," that is too strong to be explained by the Yarkovsky effect or solar radiation pressure.[3] What made this notable was that, while this is typical of comets, observations by the Spitzer Space Telescope did not find any cometary activity.[3] [1] As astrophysicist Darryl Seligman put it, "It moved like a comet, but didn't have the classic coma, or tail, of a comet."
Since the unexplained acceleration is away from the direction of the sun, the scientists hypothesized that the acceleration is produced by outgassing from the sunlit side of the surface.[3]
A team of astronomers began searching the asteroid belt for more objects that behaved the same way. By 2024, astronomers had identified 14 asteroids that moved like comets.[4] These were divided into two families.[4] The outer family was made up of larger asteroids, measuring hundreds of meters or more, and seemed to reflect more light. They had larger orbits, resembling those of Jupiter-family comets, which pass near Jupiter's orbit at aphelion. The inner family asteroids were smaller, all 50 meters or less in diameter, and had more circular orbits.
Exploration
[edit ]Japan's Hayabusa2 mission was already set to rendezvous with asteroid 1998 KY26 , before astronomers had even identified it as a dark comet. The encounter will take place in July 2031. Astronomers see this as an excellent opportunity to learn more about why dark comets behave the way they do.[4]
References
[edit ]- ^ a b Seligman, Darryl Z. (2023年02月15日). "Dark Comets? Unexpectedly Large Nongravitational Accelerations on a Sample of Small Asteroids" . The Planetary Science Journal. 4, Number 2. doi:10.3847/PSJ/acb697 . hdl:10150/674306 . Retrieved 2025年07月13日.
- ^ Seligman, Darryl Z.; Farnocchia, Davide; et al. (2024年12月09日). "Two distinct populations of dark comets delineated by orbits and sizes". Proceedings of the National Academy of Sciences of the United States of America. 1. doi:10.1073/pnas.2406424121 . PMC 11665882 . Retrieved 2025年07月13日.
- ^ a b c Hoang, Thiem; Loeb, Abraham (2020年08月17日). "Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 ('Oumuamua)". The Astrophysical Journal Letters. 899, Number 2. arXiv:2006.08088 . doi:10.3847/2041-8213/abab0c . Retrieved 2025年07月13日.
- ^ a b c "Dark Comets". Scientific American (May 2025): 58–63. 2025年05月01日. Retrieved 2025年07月13日.
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